Centrifugal pumps are often used to raise liquids from a lower level to a higher level, such as raising waste water from a sump. If the delivery from the pump is not regulated, the centrifugal pumps must be operated intermittently, which is, especially in the case of sewage plants, undesirable. Various methods of regulating delivery are known. For example, delivery may be reduced by throttling the outlet or pressure line, which causes increased co-rotation and corresponding reduction in fluid flow through the pump.
Another method of reducing delivery utilize a suction tube so placed with respect to the inlet of the pump as to induce increased suction through the suction tube in a manner to induce increased rotational fluid flow an attendent reduction in pump delivery as the level of the liquid drops. In practice, the pump delivery may be reduced to about half the maximum delivery; however, in many cases it is desirable to obtain regulation over the entire range, between zero output and maximum delivery.